As a reinforcing element for rubber products such as pneumatic tires, hoses and industrial belts, a metal cord has been popularly used from the viewpoints of good reinforcing effect and the like. In a rubber-cord complex such as a rubber product reinforced with such a metal cord, the surface of element wires of the cord is plated with brass containing copper and zinc in order to enhance the adhesive property of the metal cord to the rubber. Typically, this brass plating is performed in such a manner as sequentially forming a copper plated layer and a zinc plated layer on the surface of an element wire and then subjecting them to thermal diffusion to give an alloy of the two metals.
It is known that the adhesive property between the brass plated layer and the rubber is revealed by formation of an adhesion reaction layer between the brass plated layer and the rubber through a cross-linking reaction which occurs between copper in the brass plated layer and sulfur incorporated into the rubber during vulcanization of the rubber.